Re: Microoptimization - variables inside or out of the loop block?

From:

Eric Sosman <Eric.Sosman@sun.com>

Newsgroups:

comp.lang.java.help

Date:

Wed, 21 Oct 2009 10:53:03 -0400

Message-ID:

<1256136768.269738@news1nwk>

Ivan Voras wrote:

Hi,

Is there any point in trying to help the JVM by replacing this kind of
code:

for (i = 0; i < a_lot; i++) {
   doSomething();
   LargeObject o = getNextLargeObject();
   doSomethingElse(o);
   ...
}

with:

LargeObject o;
for (i = 0; i < a_lot; i++) {
   doSomething();
   o = getNextLargeObject();
   doSomethingElse(o);
   ...
}

In other words, does variable declaration inside a loop block result in
extra processing per loop iteration that wouldn't be there if it's
declared only once, outside the block?

The effect, if any, is probably small enough to be difficult
to measure. At a guess (and without having made any of those
difficult measurements), I'd imagine that the second form might
be very slightly slower: By extending the scope of `o' past the
end of the loop it might force Java to use one more stack slot
than it would have otherwise, and it might prevent or delay the
garbage collection of the final LargeObject.

I'd recommend against letting concerns of this kind drive
your implementation decisions.

--
Eric.Sosman@sun.com